What is the function of the MED (Multi-Exit Discriminator) attribute in BGP?

The BGP Multi-Exit Discriminator (MED) is an optional, non-transitive attribute used to influence how a neighboring Autonomous System (AS) chooses an entry point into the advertising AS. When an AS has multiple connections to another single AS, it can set the MED attribute on its advertisements to suggest a preferred path for incoming traffic. The neighboring AS will typically prefer the route with the lower MED value, all other factors being equal. This allows an AS to control how traffic enters its network from a specific neighbor.

A. Prioritizing routes within the same AS is the primary function of the Local Preference attribute, not MED.

C. Tracking the path of ASes a route has traversed is the function of the ASPATH attribute, which is also BGP's primary loop prevention mechanism.

D. Specifying the next hop is the function of the NEXTHOP attribute, which determines the immediate next router in the path.

1. Juniper Networks Documentation

Junos OS 23.4

"BGP Attributes—An Overview"

Section: "MULTIEXITDISC". States

"The MED attribute is a hint to an external neighbor about the preferred path into an AS that has multiple entry points."

2. RFC 4271

"A Border Gateway Protocol 4 (BGP-4)"

Section 5.1.4

"MULTIEXITDISC". It specifies

"This attribute MAY be used on external (inter-AS) links to discriminate among multiple exit or entry points to the same neighboring AS."